Some integrated circuits have a fluxgate magnetometer. A fluxgate magnetometer consists of a small, magnetically susceptible core wrapped by two coils of wire. An alternating electrical current is passed through one coil driving the coil through alternating cycles of magnetic saturation. The constantly reversing magnetic field in the core induces an electrical current in the second coil. In a magnetically neutral background, the input and output currents match. However, when the core is exposed to a background magnetic field, it will be more easily saturated in alignment with that field and less easily saturated in opposition to it. Hence the alternating magnetic field, and the induced output current, will be out of step with the input current. The extent to which they are out of step depends upon the strength of the background magnetic field. Typically the current in the output coil is integrated yielding an output analogy voltage which is proportional to the magnetic field.
Integrating the fluxgate magnetometer into the integrated circuit fabrication process requires forming a magnetic core of high permeability material such as permalloy (NiFe), with a thickness of a micron or more. To improve electrical properties the magnetic core may be formed of multiple, alternating layers of high permeability material and a dielectric material such as aluminium nitride (AlN). Finding an etch that etches a thick stack of multiple layers of two different materials with good dimensional control, with good profiles, and with insensitivity to material properties such as grain size is challenging.